Effects of Biochar on Soil Nutrient Retention and Microbial Communities in Vegetable Cropping Systems

Final Report for GNC13-166

Project Type: Graduate Student
Funds awarded in 2013: $9,367.00
Projected End Date: 12/31/2013
Grant Recipient: Iowa State University
Region: North Central
State: Iowa
Graduate Student:
Faculty Advisor:
Dr. Ajay Nair
Department of Horticulture, Iowa State University
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Project Information


Field experiments investigating the fitness of biochar as a soil amendment in vegetable cropping systems were carried out in 2012 and 2013 at the Iowa State University Horticulture Research Station in Gilbert Iowa.  Biochar was added at four rates (0.0, 1.1, 2.2 and 4.5 kg.m-2) to two plots both a loam and sand soil. Biochar did not have an effect on yields of marketable fruit in either soil. Biochar did however reduce yields of unmarketable fruit especially fruit with blossom end rot and sun scald. Biochar also affected soil nitrogen in soil tests, as well as nitrates in the leachate samples taken throughout the season. However, these differences were not consistent between soil types or years.


In the Midwest many new farmers are turning to vegetable production on small acreages. This trend is in large part due to high land and equipment prices, associated with modern row crop farming, as well as an increase demand for local foods by consumers. This increase interest in producing vegetables on small acreages has even been recognized by organizations like SARE and National Center for Appropriate Technology (NCAT). In fact, much NCAT literature is aimed at educating and preparing beginning farmer to be profitable while maintaining and/or improving their soils and land. The NCAT also recognizes that these new farmers range in age from young adults just starting out to retirees looking for a post retirement career. These new farmer also have various levels of experience and expertise. One trait these farms often share is they are quick to adopt new ideas and technologies as they come out. One such technology that is gaining interest among small farmers includes the use of carbon as a soil amendment. This soil amendment is referred to as biochar. Ideally biochar would be sourced locally from the byproducts of biomass that has been processed using pyrolysis to extract biofuels and energy. Pyrolysis is the process of thermally decomposing biomass in a low oxygen environment. Pyrolysis yields bio-oils, syngas, and heat, as well as black carbon (Bridgwater, 2003).

A search on the internet will return many self-proclaimed experts touting biochar as a benefit to both the environment and plant growth. Many laboratory trials have indeed shown possible benefits with the addition of biochar in temperate soils.  There is, however, little evidence that these benefits carry over to field trials (Jones et al., 2012).  In fact, there is evidence to indicate high levels of biochar may actually reduce plant growth because of changes in nutrient availability (Mikan & Abrams, 1996).  Many of the field studies showing benefits with additions of biochar have been performed in poor tropical soils and soils with otherwise low productivity (Atkinson et al., 2010).  It is therefore important that research into biochar’s effects on plants and soils must consider possible differences between locations and soil types.  Atkinson et al. (2010) noted that soils with differing physical and chemical properties will likely react differently to additions of biochar.  They also conclude that biochar may better improve soils that are degraded and otherwise unproductive.

Project Objectives:

The aim of this project was to investigate how biochar affects plant growth and yield, as well as soil nutrients and health, in the first two years of production. The first objective was to look at what affect, positive or negative, biochar would have on plant fitness and fruit yield. Plant fitness included plant growth, chlorophyll, and fruit yield. The second objective aimed to record the effect biochar had on soil quality including nutrient retention.


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  • Dr. Ajay Nair


Materials and methods:

Plot preparation. This experiment was conducted at the Iowa State University Horticulture Research Station in Gilbert, IA.  Two soil types were utilized: the first was a Clarion loam soil on a 2% – 6% slope, second was an Anthrosol consisting of pure sand and pea gravel base.  Biochar with a particle size between 0.54 mm and 2.38 mm was acquired from a commercial charcoal production company.  Biochar treatments (subplot) were established on 08 May 2012.  Application of biochar to 6.1 m x 6.1 m plots at four rates, 0.0, 1.1, 2.2 and 4.5 kg.m-2, following application fields were tilled to a depth of 15 cm. Two beds were established, one with black embossed plastic mulch, and the other as bare soil.

‘Paladin’ pepper seedlings were transplanted in double rows spaced 30 cm apart with 38 cm between plants.  Soil samples were obtained before addition of biochar in 2012, as well as before application of plastic mulch in 2013.  Half of the total N-P-K requirements were broadcast within the rows just before applying plastic mulch.  On the bare soil fertilizer was applied and raked in by hand. The other half of the N-P-K was applied weekly through the irrigation system at a final concentration of 200 mg-N/L.


Plant Data.  Yield data (loam field) were collected over seven harvests in both 2012 and 2013. In the sand field, 4 harvests were conducted in 2012, and six in 2013.  Peppers were sorted into marketable and non-marketable fruit.  USDA standards were used to classify marketable fruit (USDA-AMS, 2005).  Non-marketable categories included: 1) Blossom end rot (BER), and 2) sunscald, and 3) all other non-marketable problems, including, but not limited to, size, shape, and insect damage. 


Soil data.  Soil samples were taken during and after each growing season.    Soil samples were air dried then sieved through a 2-mm mesh before being analyzed for NH4-N, NO3-N, P and K concentrations.  Inorganic N was extracted using a 2 M KCl solution and analyzed using injection technology.  Extractions were sent to the Iowa State University Soil and Plant Analysis Laboratory for analysis. 


Lysimeter data.  Lysimeters, model 1900 soil moisture samplers were placed in two of the replications in each field to test for nutrient leaching.  Water from lysimeters was collected weekly throughout the growing season.  Samples were analyzed for NO3-N at the Soil and Plant Analysis Laboratory, Iowa State University.

Research results and discussion:

Fruit yield and quality

            Biochar did not affect marketable pepper production; however, biochar did affect non-marketable fruit.  There was, however, a differing non-significant numerical trend in both fields (Tables 1 and 2).  The trend during the first year was a decrease in marketable fruit with increasing rates of biochar, whereas the trend reversed in the second year and marketable fruit slightly increased with higher rates of biochar. Biochar’s effect on non-marketable fruit differed between the loam (Table 1) and sand (Table 2) soils, as well as between seasons. In the loam soil non-marketable fruit yield was similar to the marketable fruit and no differences were recorded in relationship to biochar (Table 1).  However, in the sand field in 2012, non-marketable yields decreased with increasing rates of biochar (Table 2).  Conversely we recorded an increase in non-marketable fruit in the sand field in 2013. The rate of peppers with BER symptoms decreased with greater biochar application rates in the loam field in 2012 (Table 1); however, differences were not seen in 2013.  The BER rate was not decreased in the sand field with increasing biochar rates (Table 2).  Over all there were fewer incidents of BER in both loam and sand soils in 2013 verses 2012, and neither soil had any differences in numbers of fruit with BER according to biochar application rates.  This might have been due to the cooler growing season in 2013. In each season black plastic mulch decreased pepper productivity in both loam (Table 1) and sand (Table 2) soils.


Soil nutrients

Soil sample extractions

            Biochar concentration affected soil nutrients differently in loam and sandy soil. The bare soil had a weak positive linear relationship whereas the plastic mulch did not (Fig. 1).  The loam plot had a reduced amount of NH4-N under plastic mulch as compared to bare soil (Tables 3 and 4), while an increase was seen in the sand field in the end of season sample in 2013 (Table 6).

Biochar reduced the amount of NO3-N that could be extracted in 2012 (Tables 3 and 5).  In the loam soil the reduction was only recorded at the end of the first season (Table 3), whereas a reduction was seen in both the middle and end of first season sandy soil samples (Table 5).  Reduction in nitrates had a linear relationship to the amount of biochar added. There was no effect on NO3-N in either soil in 2013 (Tables 4 and 6).  Black plastic mulch increased retention of nitrates in the loam soil (Table 3 and 4).  This trend was also recorded in the sand field in 2013 (Table 6).


Leachate NO3-N concentration

Nitrates were reduced in the leachate with increased additions of biochar in the loam plot samples that were taken earlier in the season in 2012. (Fig. 2A). Nitrates were hardly detectable later in the season. This same trend was seen in the sand field in 2013 (Fig. 3B). There was a greater or equal NO3-N concentration in the 4.5 kg.m-2 biochar treatment early in 2013 (Fig. 2B). No conclusive trend was seen in nitrates leached from the sand field in 2012 (Fig. 3A)

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Information from this research has been shared with growers at Fruit and Vegetable Field Day held annually at the Horticulture Research Station in Gilbert, IA in 2012, 2013 and 2014. More detailed information on this study is available in my graduate theses titled “Biochar’s fitness as an amendment in bell pepper transplant and field production” available at http://lib.dr.iastate.edu/etd/14040/.  Articles, generated by this research, will be submitted to HortScience and HortTech for publication as they are completed.

Project Outcomes

Project outcomes:

Biochar had little to no effect on marketable fruit production in either soil type. However, there was a decrease in total non-marketable fruit with increasing levels of biochar. Biochar affected height and chlorophyll content in both loam and sand fields.  It is also important to note that black plastic mulch decreased fruit production and plant growth.  Soil nutrients were affected by additions of  biochar, however these results were not consistent between loam and sand soil types.  Leaching of NO3-N was decreased with the addition of biochar in the loam field (2012), as well as in the sand field (2013).  This decrease in NO3-N in the leachate coincided with decreases in leaf chlorophyll content indicating a possible reduction in the extractability of nitrates from the soil. We speculated that fresh biochar additions interact with microbial communities and nutrients in the soil the first year, reducing plant productivity, as indicated by Bruun et al. (2011).

The reduction in overall non-marketable fruit was due to a reduction in sun scald and BER was reduced in the loam field in 2012 with increasing levels of biochar (Table 1).  BER is a physiological disorder that appears when pepper plants are unable to translocate adequate calcium to the fruit.  BER can occur from insufficient amounts of calcium in the soil, or as a result of insufficient soil moisture (Bosland & Votava, 2000).  Additions of biochar have been reported to increase extractable calcium in temperate loam soils (Laird, et al., 2010). Therefore any reductions in BER may have been due to the extra calcium attributed to the addition of biochar. 

We hypothesized biochar would increase the ability of both soils to capture nutrients.  The only difference in NH4 was in the loam plot in 2013 where extractable NH4-N increased with increasing biochar additions in the bare soil treatment only (Fig. 1).  We speculated that the plastic mulch may have increased the soil temperature leading to increased microbial activity and nitrification of ammonium.  This is consistent with the review by Deluca et al. (2009) except they indicated more nitrification in boreal soils where nitrification rates are generally low.  Complexity in microbial reaction could also explain the reduction in nitrate leaching from the loam soil (2013) and sand soil (2013).  It is possible that nitrogen was reduced in the leachate because it was used by microorganisms in the soils (Theis & Rilig, 2009).  This may also explain the reduction of chlorophyll in the respective fields. Chlorophyll content, as indicated by SPAD values, has been shown to be strongly correlated with petiole N concentration (Sexton & Carroll, 2002).

Our research indicated that biochar does not change marketable fruit production when added to either a productive agricultural soil, or nonproductive sand soil.  This is consistent with other field trials in temperate locations (Jones et al., 2012; Revell et al., 2012a).  Our study did, however, provide evidence that overall productivity of the plants may be reduced in the first year after application. However the reductions were mostly attributable to decreases in non-marketable fruit.  Atkinson et al. (2012) proposed that biochar additions to freely draining and degraded soils may show greater benefits over additions to productive soils in temperate regions. However, our study did not confirm this hypothesis. One of the reasons we may not have seen an improvement over the control is that we provided water when needed, and fertilized during some of those irrigation events.  It is also important for growers to consider that biochar is not inert when added to the soil. Biochar undergoes physical, chemical, and microbial changes when added to soils (Hammes & Schmidt, 2009).  Therefore continued and long term research is needed.


Areas needing additional study

More and continuing research is needed on how biochar interacts with soils, especially when applied at greater rates. It would also be good to look at how biochar interacts with irrigation, in field conditions. It is also important that research be continued at these locations to demonstrate how biochar affects soil and plant health long term after application.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.